I did two away rotations, and I think they were extremely helpful for both getting an interview, and getting to know the program. I already knew how to use the EMRs, but I didn't feel too disadvantaged not knowing the people, hospital and place. Perhaps it was because these are very large programs that often have rotators, but I thought it was easy to really "show your stuff" and do really well clinically.
Most importantly, as someone mentioned above, it was a month to really get to know the program. I loved both of the places I went. But, I had a friend who after spending a month at what she thought was her number one, end up disliking it and not even applying there. I agree with the poster above- it's really hard to get a good idea of programs based on the interview day! They do a wonderful job of selling the program and recruiting, so spending a month there is very valuable when it comes time to rank places.